1. Field of the Invention
The present invention relates to a method of polishing a thin film formed on a substrate, and more particularly to a polishing method which can properly polish a thin film by determining a physical property and a thickness of the thin film utilizing ellipsometry.
2. Description of the Related Art
Ellipsometry is a method of determining the refractive index of a specimen by emitting polarized light to a flat surface of the specimen and measuring a change in the polarization state of reflected light. Ellipsometry is also used for determining a thickness of a thin film formed on a surface of a specimen.
Fabrication processes of an IC chip include a process of forming thin films of several kinds of materials on a substrate (e.g., Si substrate) and a process of removing the thin films using CMP (i.e., chemical mechanical planarization). In a case of a copper thin film, for example, an oxide film such as CuO or CuO2 is likely to be formed on a surface of the copper thin film. Therefore, while performing a CMP process, a pressing force should be properly adjusted when pressing the oxide film against a polishing pad and when pressing a highly pure copper after the oxide film is removed. Accordingly, when performing a CMP process, it is desirable to know the thickness of the oxide film in advance so as to set a proper polishing condition based on the thickness of the oxide film.
Ellipsometry is conducted by emitting linearly polarized light to a thin film, and measuring a phase angle Δ of elliptically polarized light reflected from the thin film and a tangent Ψ that is given by a magnitude ratio of amplitude of the elliptically polarized light. If an optical constant (i.e., a complex refractive index) of a material forming the thin film is known, the thickness of the thin film can be determined based on the relationship between the optical constant and Δ and Ψ as measured.
However, with respect to the copper thin film formed on the substrate in an IC chip fabrication process or other process, it has been found that it is not appropriate to determine the film thickness by such ellipsometry using the optical constant of copper which is published in handbooks, documents, and the like. FIG. 1 shows this fact. In FIG. 1, a mark ● shows Δ and Ψ measured by ellipsometry. In this case, the measurement of Δ and Ψ is performed after a copper thin film on a substrate has been planarized by CMP to remove an oxide film from the thin film and the thin film has then been cleaned. Marks ◯ show time-series data of Δ and Ψ measured by ellipsometry when oxidization proceeds after the planarization.
On the other hand, marks ♦, ▴, and ▪ show Δ and Ψ calculated using optical constants (i.e., complex refractive indexes) published in handbooks and documents. These optical constants are given with respect to copper having a clean surface with no oxide film thereon. As can be seen from FIG. 1, there is an obvious difference between the values of Δ and Ψ which are actually measured with respect to the copper film and the oxide film on the substrate and the values of Δ and Ψ calculated using the optical constants which are commonly known. Specifically, FIG. 1 shows that the thickness of the thin film cannot be accurately determined from the commonly known complex refractive index and the values of Δ and Ψ measured by ellipsometry. Japanese laid-open patent publication No. 07-193033 discloses a related art of this technical field.